1. Field of the Invention
The present invention relates to color processing for color-converting color data of the first color gamut into that of the second color gamut.
2. Description of the Related Art
Recently, opportunities are increasing to input an image by an input device such as a digital camera, color scanner, or the like, to display the image by using an image display device such as a CRT, LCD, or the like, and to output the image by an output device such as a color printer or the like. In this case, color matching is generally performed to absorb differences in color impression in image caused by differences between a color gamut (to be referred to as an “input gamut” hereinafter) of a color space in which the input device records an image and a color gamut (to be referred to as an “output gamut” hereinafter) of the display device or output device. More specifically, color matching is performed to map an input gamut into an output gamut, thereby correcting differences in color impression in image between the devices.
One method of color gamut mapping is referred to as colorimetric mapping which colorimetrically matches colors. Colorimetric mapping maps not colors in a common region of an input gamut and output gamut but colors in their non-common region. When the output gamut is narrower than the input gamut, colors outside the output gamut are mapped into the boundary surface of the output gamut. More specifically, colorimetric mapping is a mapping method devised to match absolute color values on a colorimetric space.
Assume that the white point of an input device is different from that of an output device. In this case, when colorimetric mapping is performed for a color which is white in the input device, the color is sometimes reproduced unnaturally by the output device. In order to avoid this, a mapping method called relative colorimetric mapping is available. In this method, before performing colorimetric mapping, the color gamuts of the input and output devices are corrected to match their white points, so that the white point of the input device is reproduced as that of the output device. As a color gamut correction (white point correction) method in relative colorimetric mapping, for example, a technique described in US Patent Application Publication No. 2002/0097907 is available. When the white point of an input device is different from that of an output device, this technique scales the white point of the output device in an XYZ space to match the white points of the input and output devices.
However, when color gamut mapping described in US Patent Application Publication No. 2002/0097907 is performed, an output gamut largely shifts in a colorimetric space, as shown in FIG. 1 in which the uncorrected color gamut is represented by the solid lines and the corrected color gamut is represented by the broken lines. For this reason, color impression in image of the input image largely changes after relative colorimetric mapping. Therefore, the original purpose of colorimetric mapping to match absolute color values in a colorimetric space is not achieved.
The above-described problem also arises when black points of input and output devices are matched. In other words, the above-described problem arises when performing colorimetric matching by matching a specific color point including the white or black point on the neutral axis in the color gamut of an input device to that of an output device.